Magnetic recording medium

ABSTRACT

A magnetic recording medium having good dispersibility, dispersion stability of a magnetic paint and electromagnetic characteristics, and excellent running durability and still characteristics, comprises a non-magnetic support and a magnetic layer formed thereon which is mainly composed of a magnetic powder and a binder, wherein said binder is a mixture of 
     i) a vinyl chloride copolymer having amine-modified vinyl units and acidic functional group-containing vinyl units in a molecule, 
     ii) a polyurethane resin having a number average molecular weight of 5000 to 25000 (Mn 1), and 
     iii) a polyurethane resin having a number average molecular weight of higher than 25000 to 100000 (Mn 2).

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic recording medium having amagnetic layer obtained by applying a magnetic paint (magnetic coatingmaterials) containing a ferromagnetic powder used for recording andreproducing images, voices and data.

Previously, some magnetic recording media have been known which havemagnetic layers formed by applying magnetic paints comprising magneticpowders and binders to non-magnetic supports, said binders beingcombinations of a plurality of resins.

For example, magnetic recording media have been known such as "amagnetic recording medium comprising a non-magnetic support and amagnetic layer formed thereon comprising a ferromagnetic metal powderand a binder, wherein said binder comprises three kinds of binder resinsdifferent from one another in glass transition temperature (Tg), and amixed resin composition of these three kinds of resins has a glasstransition temperature (Tg) of 30° C. to 47° C." (Japanese PatentUnexamined Publication No. 3-286419), "a magnetic recording mediumcomprising a non-magnetic support and a magnetic layer formed thereoncomprising a ferromagnetic powder and a binder, wherein said binderconsists of at least two kinds of polyester-polyurethane resinsdifferent from each other in glass transition temperature, and has aglass transition temperature of 10° C. to 60° C. as a whole" (JapanesePatent Unexamined Publication No. 5-307734), and "a magnetic recordingmedium comprising a non-magnetic support and a magnetic layer formedthereon comprising a ferromagnetic powder and carbon black dispersed ina binder, wherein said binder contains a copolymer having amine-modifiedvinyl units and acidic functional group-containing vinyl units, and saidcarbon black has a specific surface area of 90 m² /g to 330 m² /g and aDBP oil absorption of 45 ml/100 g to 120 ml/100 g" (Japanese PatentUnexamined Publication No. 7-153055).

Japanese Patent Unexamined Publication No. 3-286419 described abovediscloses that the three kinds of resins different from one another inglass transition temperature are used as the binder, thereby obtainingthe magnetic recording medium excellent in surface properties andmechanical strength and improved in durability. Further, Japanese PatentUnexamined Publication No. 5-307734 discloses that at least two kinds ofpolyester-polyurethane resins different from each other in glasstransition temperature are used as the binder, thereby resulting animprovement in electromagnetic characteristics. However, both of them donot describe vinyl chloride copolymers in detail, and have a problemwith respect to the dispersibility of paints. Japanese Patent UnexaminedPublication No. 7-153055 is directed to the use of the copolymer havingamine-modified vinyl units and acidic functional group-containing vinylunits in a molecule as the binder, which provides the magnetic recordingmedium excellent in dispersibility. However, it is silent on the use oftwo kinds of polyurethane resins.

For conventional techniques, problems are left unsolved with regard tothe dispersibility and the dispersion stability of the magnetic paints.As a result, the electromagnetic characteristics are deteriorated,resulting in a problem with respect to running durability. It has beentherefore desired to provide a magnetic recording medium excellent inboth the electromagnetic characteristics and the running durability.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems, the present inventorshave conducted intensive investigations. As a result, the inventors havediscovered that the above-mentioned problems can be solved by the use ofa binder comprising a mixture of a vinyl chloride copolymer having aspecified functional group and two kinds of polyurethane resins eachhaving specified molecular weights different from each other, thuscompleting the present invention.

That is, the present invention provides:

(1) a magnetic recording medium comprising a non-magnetic support and amagnetic layer formed thereon which is mainly composed of a magneticpowder and a binder, wherein said binder is a mixture of

i) a vinyl chloride copolymer having amine-modified vinyl units andacidic functional group-containing vinyl units in a molecule,

ii) a polyurethane resin having a number average molecular weight of5000 to 25000 (Mn 1), and

iii) a polyurethane resin having a number average molecular weight ofhigher than 25000 to 100000 (Mn 2);

(2) the magnetic recording medium described in (1), wherein said vinylchloride copolymer has an average degree of polymerization of 200 to800;

(3) the magnetic recording medium described in (1), wherein the contentof the amine-modified vinyl units contained in the vinyl chloridecopolymer is 0.03% to 2.3% by weight, and the content of the acidicfunctional group-containing vinyl units is 0.05% to 1.8% by weight;

(4) the magnetic recording medium described in (1), wherein a primaryamine, a secondary amine or a tertiary amine is contained as theamine-modified vinyl unit, and a carboxyl group, a sulfonic acid group,a sulfuric acid group, a phosphoric acid group or a phosphonic acidgroup is contained as the acidic functional group-containing vinyl unit;

(5) the magnetic recording medium described in (1), wherein saidpolyurethane resin having a number average molecular weight of 5000 to25000 (Mn 1) is a polyester-polyurethane resin, a polyetherpolyurethaneresin or a polycarbonatepolyurethane resin;

(6) the magnetic recording medium described in (1), wherein saidpolyurethane resin having a number average molecular weight of higherthan 25000 to 100000 (Mn 2) is a polyesterpolyurethane resin, apolyetherpolyurethane resin or a polycarbonatepolyurethane resin; and

(7) the magnetic recording medium described in (1), wherein said vinylchloride copolymer is contained in an amount of 20% to 80% by weightbased on the total amount of the binder, said polyurethane resin of Mn 1is contained in an amount of 10% to 50% by weight, said polyurethaneresin of Mn 2 is contained in an amount of 10% to 50% by weight, and thetotal amount of said polyurethane resin of Mn 1 and, said polyurethaneresin of Mn 2 is 20% to 80% by weight.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, a vinyl chloride copolymer havingamine-modified vinyl units and acidic functional group-containing vinylunits in a molecule is used in combination with two kinds ofpolyurethane resins, each having the specified molecular weightsdescribed above as a binder, thereby being able to form a paint improvedin dispersibility and paint stability, and to obtain a magneticrecording medium improved in electromagnetic characteristics and runningdurability.

The acidic functional group-containing vinyl unit contained in the vinylchloride copolymer used in the present invention is a vinyl unitcontaining at least one kind of functional group such as a carboxylgroup, a sulfonic acid group, a sulfuric acid group, a phosphoric acidgroup or a phosphonic acid group. The vinyl unit may be any as long asit has the above-mentioned functional group. The carboxylgroup-containing vinyl groups include, for example, maleic acid, fumaricacid, acrylic acid and methacrylic acid. The sulfonic acidgroup-containing vinyl groups include sodium styrenesulfonate, sodiumvinylsulfonate, sodium methacrylsulfonate and sulfoethyl acrylate. Ofthese, particularly preferred are maleic acid or fumaric acid having acarboxyl group, sulfoethyl acrylate or sodium methacrylsufonate having asulfonic acid group, and 2-acid phosphooxyethyl methacrylate having aphosphoric acid.

The amine-modified vinyl unit contained in the vinyl chloride copolymerused in the present invention contains an amino group. The amino groupmay be either unsubstituted or substituted. The amino groups can beintroduced by treating the vinyl chloride copolymers containing theabove-mentioned acidic functional group-containing vinyl units withvarious amines described later. The amine-modified vinyl units mayfurther contain the various functional groups illustrated as thefunctional groups contained in the above-mentioned acidic functionalgroup-containing vinyl groups.

In the present invention, the content of the amine-modified vinyl unitsin the above-mentioned copolymer contained in the binder is preferably0.03% to 2.3% by weight, more preferably 0.05% to 2.0% by weight, andmost preferably 0.5% to 1.5% by weight. Too high a content of theamine-modified vinyl units is liable to result in deterioration of thestability of a paint, whereas too low a content thereof tends to resultsin deterioration of the dispersibility. The content of the acidicfunctional group-containing vinyl units is preferably 0.05% to 1.8% byweight, more preferably 0.1% to 1.5% by weight, and most preferably 0.5%to 1.5% by weight. Too high a content of the acidic functionalgroup-containing vinyl units is liable to cause an increase inthixotropic properties of a paint, resulting in a reduction in surfacesmoothness of a magnetic layer coated. On the other hand, too low acontent thereof tends to deteriorate the stability of a paint to producean aggregate, resulting in deterioration of surface properties of amagnetic layer.

The vinyl chloride copolymer having the above-mentioned amine-modifiedvinyl units and acidic functional group-containing vinyl units in amolecule used in the present invention contains vinyl chloride, a vinylalkylcarboxylate ester or vinyl alcohol.

Of these constituent units, vinyl acetate is preferably used as thevinyl alkylcarboxylate in respect to the cost of raw materials. Vinylpropionate and vinyl versatate can also be used. Further, vinyl alcoholcan be obtained by saponifying an alkylcarboxylic acid moiety of theabove-mentioned vinyl alkylcarboxylate ester. The content of the vinylalkylcarboxylate ester is preferably 0.5% to 10% by weight, and morepreferably 1.0% to 5.0% by weight. The content of vinyl alcohol ispreferably 0% to 8.0% by weight, and more preferably 3.0% to 6.0% byweight. It is preferred that the remainder excluding these issubstantially a vinyl chloride. If the content of the vinylalkylcarboxylate ester is too high, the affinity for a magnetic powderis liable to be lowered when it is used in a paint, resulting indeterioration of the dispersibility. On the other hand, too low acontent thereof tends to cause deterioration of the solubility of theresins. If the content of vinyl alcohol is too high, the saponificationrate is increased and likely to cause a reduction in heat stability ofthe copolymer, leading to deterioration of shelf-stability.

The average degree of polymerization of the copolymer is preferably 200to 800, and more preferably 300 to 500. Too high a degree ofpolymerization is liable to deteriorate the solubility anddispersibility of the copolymer, whereas too low a degree ofpolymerization tends to lower the strength of a magnetic layer toproduce scratches on a coated film of the magnetic layer, leading to anincrease in dropout.

Such a copolymer may further contain about 0.5% by weight or less ofanother monomer component such as acrylglycidyl ether or 2-hydroxymethylmethacrylate.

The copolymers used in the present invention may be produced by anyprocedure. For example, vinyl chloride, a vinyl alkylcarboxylate esterand a monomer having the functional group contained in the acidicfunctional group-containing vinyl unit illustrated above arecopolymerized by suspension polymerization, emulsion polymerization,solution polymerization or bulk polymerization. A copolymer thusobtained, an intermediate product, is saponified in the presence of analkali such as KOH or NaOH or an acid such as hydrochloric acid orsulfuric acid as a catalyst in an organic solvent such as an alcohol. Inthis case, an amine compound described later is added, followed bystirring at an appropriate temperature for a definite period of timeuntil a desired saponification rate is obtained to obtain a copolymer.The copolymer thus obtained contains vinyl alcohol.

The amine compounds used in the above-mentioned amine modificationinclude primary, secondary and tertiary amines such as aliphatic amines,alicyclic amines, aromatic amines, alkanol amines and alkoxyalkylamines. Examples thereof include methylamine, ethylamine, propylamine,butylamine, cyclohexylamine, ethanolamine, naphthylamine, aniline,o-toluidine, diethylamine, dioctylamine, di-isobutylamine,diethanolamine, methylethanolamine, di-methylethanolamine,diethylethanolamine, dibutylethanolamine, 2-methoxyethylamine,N-methylaniline, trimethylamine, triethylamine, triisobutylamine,tridecylamine, N-methylbutylamine, N-methyldiphenylamine,hexamethylenetetramine, triethanolamine, tributylamine,dimethylpropylamine, pyridine, α-picoline, β-picoline, γ-picoline,2,4-lutidine, quinoline, monoforine, sodium taurinate, potassiumsulfanilate, cetylaminesulfonic acid, diaminopropane andhexamethylenediamine. Of these, the aliphatic tertiary amines are mostpreferred in terms of the excellent dispersibility and stability of apaint.

Examples of the vinyl chloride copolymers having amine-modified vinylunits and acidic functional group-containing vinyl units in a moleculeinclude a vinyl chloride/vinyl acetate/vinylalcohol/N,N-dimethylethanolamine-modified vinyl unit/maleic acidcopolymer (91%/3.0%/5.0%/0.5%/0.5% by weight), a vinyl chloride/vinylacetate/vinyl alcohol/N,N-dimethylethanolamine-modified vinylunit/maleic acid copolymer (91%/3.0%/4.5%/1.0%/0.5% by weight), a vinylchloride/vinyl acetate/vinyl alcohol/N,N-dimethylethanolamine-modifiedvinyl unit/maleic acid copolymer (91%/3.0%/4.0%/1.5%/0.5% by weight), avinyl chloride/vinyl acetate/vinylalcohol/N,N-dimethylethanolamine-modified vinyl unit/maleic acidcopolymer (91%/3.0%/4.5%/0.5%/1.0% by weight), a vinyl chloride/vinylacetate/vinyl alcohol/N,N-dimethylethanolamine-modified vinylunit/maleic acid copolymer (91%/3.0%/4.0%/0.5%/1.5% by weight), a vinylchloride/vinyl acetate/vinyl alcohol/N,N-dimethylethanolamine-modifiedvinyl unit/sodium methacrylsulfonate copolymer (91%/3.0%/5.0%/0.5%/0.5%by weight), a vinyl chloride/vinyl acetate/vinylalcohol/N,N-dimethylethanolamine-modified vinyl unit/2-acidphosphooxyethyl methacrylate copolymer (91%/3.0%/5.0%/0.5%/0.5% byweight), a vinyl chloride/vinyl acetate/vinylalcohol/methylamine-modified vinyl unit/maleic acid copolymer(91%/3.0%/5.0%/ 0.5%/0.5% by weight) and a vinyl chloride/vinylacetate/vinyl alcohol/dimethylamine-modified vinyl unit/maleic acidcopolymer (91%/3.0%/5.0%/0.5%/0.5% by weight).

A magnetic paint improved in the dispersibility and stability of thepaint can be obtained by containing such a copolymer in the binder. Evenif a mixture of a copolymer having the amine-modified vinyl units and acopolymer having the acidic functional group-containing vinyl units isused in place of the above-mentioned copolymer having both theamine-modified vinyl units and the acidic functional group-containingvinyl units in a molecule, an effect equivalent to that obtained for theabove-mentioned copolymer used in the present invention can not beobtained.

The vinyl chloride copolymer having the amine-modified vinyl units andthe acidic functional group-containing vinyl units is contained in anamount of 20% to 80% by weight based on the total amount of the binder.Less than 20% by weight reduces the strength of a coated film todeteriorate the running stability, resulting in a tendency to causetroubles during running, whereas exceeding 80% by weight lowers thecalendaring processability which is liable to deteriorate theelectromagnetic characteristics.

In the present invention, the binder contained in the magnetic layercomprises polyurethane resin 1 having a number average molecular weight(Mn) of 5000 to 25000 (Mn 1), preferably 8000 to 23000, and polyurethaneresin 2 having a number average molecular weight of less than 25000 to100000 (Mn 2), preferably 30000 to 80000, as essential ingredients.

In the present invention, the inclusion of polyurethane resin 1 havingthe above-mentioned Mn 1 improves the dispersibility of the magneticpaint to obtain the excellent electromagnetic characteristics. Further,the inclusion of polyurethane resin 2 having the above-mentioned Mn 2improves the strength of a coated film, and reduces powder falling andhead adhesion to obtain the excellent running durability.

Too low an Mn 1 of polyurethane resin 1 deteriorates the dispersibilityof the magnetic paint. Further, the strength of the coated film isdecreased to deteriorate the running stability, resulting in a tendencyto cause troubles during running. On the other hand, too high an Mn 1tends to deteriorate the dispersibility of the magnetic paint, whichcauses deterioration of the electromagnetic characteristics.

Too low an Mn 2 of polyurethane resin 2 deteriorates the runningstability, resulting in a tendency to cause troubles during running,whereas too high an Mn 2 deteriorates the dispersibility of the magneticpaint, resulting in deterioration of the electromagnetic characteristicssuch as reproduction output.

The content of polyurethane resin 1 described above is 10% to 50% byweight based on the binder, and preferably 15% to 45% by weight. Thecontent of polyurethane resin 2 is 10% to 50% by weight based on thebinder, and preferably 15% to 45% by weight.

Too high a content of polyurethane resin 1 in the binder decreases thestrength of the coated film to deteriorate the running stability,resulting in a tendency to cause troubles during running, whereas toolow a content deteriorates the dispersibility of the magnetic paintwhich is liable to deteriorate the electromagnetic characteristics.

Too high a content of polyurethane resin 2 in the binder deterioratesthe dispersibility of the magnetic paint, which tends to result indeterioration of the electromagnetic characteristics, whereas too low acontent lowers the strength of the coated film which is liable todeteriorate the running stability.

The glass transition temperature (Tg) of polyurethane resin 1 andpolyurethane resin 2 used is preferably -50° C. to +80° C., and morepreferably -40° C. to +80° C. Too low a glass transition temperaturedeteriorates the running durability under the circumstances of hightemperature, resulting in a tendency to cause troubles such as headclogging and guide adhesion during running, whereas too high a glasstransition temperature deteriorates the calendaring processability totend to deteriorate the electromagnetic characteristics. Further, therunning durability under the circumstances of low temperature isdeteriorated.

The glass transition temperature (Tg) of these resins may be measuredwith a dynamic viscoelasticity measuring device.

Further, the total amount of polyurethane resin 1 and polyurethane resin2 is preferably 20% to 80% by weight based on the binder used, andparticularly preferably 30% to 80% by weight. The compounding ratio(weight ratio) of polyurethane resin 1 to polyurethane resin 2 ispreferably 1:5 to 5:1.

If the total content of polyurethane resin 1 and polyurethane resin 2 inthe binder is too low, the running stability is liable to bedeteriorated, or the calendaring processability is deteriorated toroughen a surface of a magnetic layer, resulting in a tendency todeteriorate the electromagnetic characteristics such as reproducingoutput. As polyurethane resins used as polyurethane resin 1 andpolyurethane resin 2, any polyurethane resins may be used as long asthey are polyurethane resins usually employed in such magnetic recordingmedia and satisfies the above-mentioned number average molecular weight.

For example, the polyurethane resins are obtained by reacting polyolssuch as polyesterpolyols, polyetherpolyols, polycarbonatepolyols,polyesterpolycarbonates, polyesterpolyethers and polycaprolactones withisocyanates, together with chain extenders and others if desired.

In order to improve the dispersibility of a powder ingredient such as aferromagnetic powder contained in a magnetic layer and the runningdurability of the magnetic layer, it is preferred that polyurethaneresin 1 and/or polyurethane resin 2 contains at least one polar group ina molecule thereof.

In this case, at least one polar group selected from hydrophilicfunctional groups such as --SO₃ M, --SO₄ M, ═PO₃ M, ═POM, --P═O(OM)₂,--OP═O(OM)₂, --COOM, --NR₃ X, --NR₂, --N⁺ R₃, an epoxy group, --OH, --SHand --CN (wherein X represents H, Li, Na, K or --NR₃, R represents analkyl group or H, and X represents a halogen atom) is preferablyintroduced by copolymerization or the addition reaction. Theintroduction of such groups improves the dispersibility of the paint,resulting in an improvement in the characteristics of the magneticrecording medium. These polar groups may exist either in main chains orin branched chains of skeleton resins.

Particularly preferred examples of the polar groups include sulfonicacid groups (--SO₃ M), carboxylic acid groups (--COOM) and phosphonicacid groups (═PO₃ M). These polar groups are preferably contained in anamount of about 0.1 to about 5 molecules per molecule of polyurethanepolymer.

The polyurethane resins used in the present invention are ones obtainedby reacting organic diisocyanates (A) with long-chain diols having amolecular weight of 500 to 5000 (B) in the presence of chain extendershaving a molecular weight of less than 500 (C).

The organic diisocyanates (A) used in the production of thepolyurethanes include 2,4-tolylene diisocyanate, 2,6-tolylenediisocyanate, p-phenylene diisocyanate, diphenylmethane diisocyanate,m-phenylene diisocyanate, hexamethylene diisocyanate, tetramethylenediisocyanate, 3,3'-dimethoxy-4,4'-biphenylene diisocyanate,2,4-naphthalene diisocyanate, 3,3'-dimethyl-4,4'-biphenylenediisocyanate, 4,4'-diphenylene diisocyanate, 4,4'-diisocyanate diphenylether, 1,5-naphthalene diisocyanate, p-xylylene diisocyanate, m-xylylenediisocyanate, 1,3-diisocyanate methylcyclohexane, 1,4-diisocyanatemethylcyclohexane, 4,4'-diisocyanate cyclohexane, 4,4'-diisocyanatecyclohexylmethane and isophorone diisocyanate.

The long-chain diols (B) used in the production of the polyurethaneresins have a molecular weight ranging from 500 to 5000, and includepolyesterdiols, polyetherdiols and polycarbonatediols.

Carboxylic acid ingredients of the polyesterdiols include aromaticdicarboxylic acids such as terephthalic acid, isophthalic acid,orthophthalic acid and 1,5-naphthalic acid, aromatic oxycarboxylic acidssuch as p-oxybenzoic acid and p-(hydroxyethoxy)benzoic acid, andaliphatic dicarboxylic acids such as succinic acid, adipic acid, azelaicacid, sebacic acid and dodecanedicarboxylic acid. In particular,terephthalic acid, isophthalic acid, orthophthalic acid, adipic acid andsebacic acid are preferred.

Further, glycol ingredients of the polyesterdiols include ethyleneglycol, propylene glycol, 1,3-propanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol,dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol,cyclohexanedimethanol, ethylene oxide adducts and propylene oxideadducts of bisphenol A, and ethylene oxide adducts and propylene oxideadducts of bisphenol A hydride.

In addition, the polyesterdiols include lactone polyesterdiols obtainedby ring-opening polymerization of lactones such as ε-caprolactone.

The polyetherdiols include polyalkylene glycols such as polyethyleneglycol, polypropylene glycol and polytetramethylene glycol.

The polycarbonatediols are long-chain diols represented by generalformula H--(O--R--OCO)_(n) --ROH, wherein R is a residue of diethyleneglycol, 1,4-butanediol, 1,6-hexanediol or bisphenol A.

The long-chain diols (B) are preferably the polyester-diols forenhancing the mechanical characteristics of the polyurethane resins. Thelong-chain diols used have a molecular weight of 500 to 5000. If themolecular weight is less than 500, the concentration of urethane groupsis increased, resulting in a reduction in flexibility and solventsolubility of the resins. On the other hand, if the molecular weightexceeds 5000, the concentration of urethane groups is decreased todeteriorate toughness and wear resistance characteristic of thepolyurethane resins.

Each of the chain extenders having a molecular weight of less than 500(C) used in the production of the polyurethane resins contains twoactive hydrogens in one molecule, and has the effect of adjusting theconcentration of urethane groups or urea groups contained in the resinsto give the toughness characteristic of the polyurethane resins.Examples of such compounds include straight chain glycols such asethylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,6-hexanediol,cyclohexanedimethanol, xylylene glycol, diethylene glycol, triethyleneglycol and ethylene oxide adducts of bisphenol A, branched glycols suchas propylene glycol, neopentyl glycol, 1,2-butanediol, 1,3-butanediol,2,2,4-trimethyl-1,3-pentanediol and propylene oxide adducts of bisphenolA, aminoalcohols such as monoethanolamine and N-methylethanolamine,diamines such as ethylenediamines, hexamethylenediamine andisophoronediamine, and water.

The polyurethane resins include, for example, the polyesterpolyurethaneresins, the polyetherpolyurethane resins and thepolycarbonatepolyurethane resins. Of these, the polyesterpolyurethaneresins are preferred. The polyesterpolyurethane resins are preferablyreaction products of polyesterdiol (1)/polyesterdiol (2)/neopentylglycol/4,4'-diphenylmethane diisocyanate. Polyesterdiol (1) is composedof, for example, terephthalic acid/isophthalicacid/5-sodiumsulfoisophthalic acid/ethylene glycol/neopentyl glycol in amolar ratio of 49/49/2/50/50, and polyesterdiol (2) is composed of, forexample, adipic acid/1,4-butanediol/neopentyl glycol in a molar ratio of100/75/25. The Tg is arbitrarily controllable by adjusting the mixingratio of polyesterdiol (1) to polyesterdiol (2) and the concentration ofurethane groups.

Examples of the polyetherpolyurethane resins include reaction productsof polyalkylene glycols/neopentyl glycol/4,4'-diphenylmethanediisocyanate, and the polyalkylene glycols include polyethylene glycol,polypropylene glycol and polytetramethylene glycol.

Examples of the polycarbonatepolyurethane resins include reactionproducts of polycarbonatediols/neopentyl glycol/4,4'-diphenylmethanediisocyanate, and the polycarbonatediols include a reaction product ofdiethyl carbonate/1,6-hexanediol and a reaction product of diethylcarbonate/ethylene glycol.

The amount of the binder used in a magnetic layer is preferably 5 partsto 40 parts by weight per 100 parts by weight of ferromagnetic powder,and particularly preferably 10 parts to 30 parts by weight. If thecontent of the binder is too low, the strength of the magnetic layer islowered, resulting in a tendency to deteriorate the running durability.On the other hand, if it is too high, the content of the ferromagneticpowder is decreased to deteriorate the electromagnetic characteristics.

As crosslinking agents for curing these binders, variouspolyisocyanates, particularly diisocyanates, can be used, andparticularly at least one of tolylene diisocyanate, hexamethylenediisocyanate and methylene diisocyanate is preferred. It is particularlypreferred to use these crosslinking agents as crosslinking agentsmodified to compounds each having a plurality of hydroxyl groups such astrimethylolpropane, or isocyanulate type crosslinking agents in each ofwhich three diisocyanate compounds are linked together. The crosslinkingagents combine with functional groups contained in the binder resins tocrosslink the resins. The content of the crosslinking agent ispreferably 10 parts to 30 parts by weight per 100 parts by weight ofbinder. In order to cure such thermosetting resins, they may begenerally heated in a heating oven at 50° C. to 70° C. for 12 hours to48 hours.

Examples of the ferromagnetic powders used in the present inventioninclude iron oxide magnetic powders such as Co compound-coated or dopetype γ--Fe₂ O₃, Co compound-coated or dope type Fe₃ O₄ and Cocompound-coated or dope type bertholide, ferromagnetic iron or alloypowders mainly composed of ferromagnetic metal elements such as α--Fe,Fe--Co, Fe--Ni, Fe--Co--Ni, Co and Co--Ni, CrO₂ magnetic powders andtabular hexagonal ferrite powders having axes of easy magnetizationperpendicular to flat plates.

Al, Si, P, Y and rare earth elements can be added to the above-mentionedferromagnetic powders to prevent sintering and to improve theparticle-size distribution, resulting in an improvement in theelectromagnetic characteristics of the magnetic recording media. In theproduction of the ferromagnetic powders, surfaces thereof may be coatedwith Al, Si, P or oxide films thereof, or may be treated with couplingagents such as Si, Al and Ti or various surfactants after the productionof the ferromagnetic powders, thereby improving the dispersibility ofthe magnetic paints and the durability of the magnetic recording media.

Lubricants, abrasives, non-ferromagnetic pigments, carbon black andorganic pigments may be added to the magnetic paints used in the presentinvention. Further, additives having the lubricating effect, theantistatic effect, the dispersing effect or the plasticizing effect areused, and silicone oils, fluorine oils, cationic surfactants, nonionicsurfactants, anionic surfactants or amphoteric surfactants can also beused.

Of various known lubricants, fatty acids and/or fatty acid esters arepreferably used as the lubricants in the present invention. The fattyacids include monobasic fatty acids each having 12 to 24 carbon atoms.The fatty acid esters include fatty acid monoesters, fatty acid diestersand fatty acid triesters synthesized from monobasic fatty acids eachhaving 10 to 24 carbon atoms and cyclic or polysaccharide-reducedalcohols prepared from monohydric to hexahydric alcohols each having 2to 22 carbon atoms, and mixtures thereof. Two or more of them may beused in combination. Hydrocarbon groups of these fatty acids and fattyacid esters may contain unsaturated bonds or may be branched. Further,the lubricants are preferably added to backcoat layers or undercoatlayers, in addition to the magnetic layers. In particular, when themagnetic layers are thin, addition of the lubricants to the undercoatlayers is effective for an improvement in still durability. Further,when the backcoat layers are provided, the lubricants can be added in alarger amount to the backcoat layers to improve the surface lubricity ofthe magnetic layers by transfer to surfaces thereof.

The abrasives and the non-magnetic pigments which can be used in thepresent invention include α-alumina, γ-alumina, θ-alumina, dichromiumtrioxide, α-iron oxide, SiO₂, ZnO, TiO₂, silicon carbide, calciumcarbonate and barium sulfate. Although any form and size of particles ofthese pigments may be used, the particle form is preferably spherical orpolyhedral, and the particle size is preferably 0.01 μm to 0.7 μm. Theymay be suitably selected according to a balance between the durabilityand friction coefficient required for the magnetic recording media andthe output at the shortest recording wavelength, and may be selected ina single system or in a mixed system. The particle-size distribution canalso be selected independently. These inorganic compounds may besuitably used in combination depending on desired characteristics of themagnetic layers, the backcoat layers and the undercoat layers.

As the carbon black which can be used in the present invention, furnacecarbon black, thermal carbon black and acetylene black can be used aloneor in combination. Further, surfaces of the carbon black particles maybe treated with lubricants or dispersing agents, or partiallygraphitized. Any size of these carbon black particles may be used, orsuitably selected according to a balance between the durability andfriction characteristics required for the magnetic recording media andthe output at the shortest recording wavelength (surface roughness), andmay be selected in a single system or in a mixed system. Theparticle-size distribution can also be selected independently. Thecarbon black may be suitably used in combination depending on desiredcharacteristics of the magnetic layers, the backcoat layers and theundercoat layers.

The non-magnetic organic powders used in the present invention, organicpigments, include acrylstyrene resin powders, benzoguanamine resinpowders, melamine resin powders, phthalocyanine pigments, polyolefinresin powders, polyester resin powders, polyamide resin powders,polyimide resin powders, hydrocarbon fluoride resin powders anddivinylbenzene resin powders.

These organic compounds may be suitably used in combination depending ondesired characteristics of the magnetic layers, the backcoat layers andthe undercoat layers.

Examples of solvents used in the present invention include ketones suchas methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone,aromatic hydrocarbons such as toluene and xylene, esters such as ethylacetate and butyl acetate, and diluents or solvents such as dioxane,tetrahydrofurane, dimethylformamide and hexane. They may be used as asingle solvent or as mixed solvents with any mixing ratio.

In the present invention, the backcoat layers may be provided.

The backcoat layers are provided for an improvement in the runningstability or prevention of static charge. The backcoat layer preferablycontains 30% to 80% by weight of carbon black. Too low a content of thecarbon black tends to decrease the antistatic effect, and is furtherliable to deteriorate the running stability. In addition, the lighttransmission rate is liable to become high, so that a problem arises ina system of detecting a tape edge by changes in the light transmissionrate. On the other hand, too much a content of the carbon black reducesthe strength of the backcoat layer, resulting in a tendency todeteriorate the running durability. The carbon black may be any as longas it is usually employed, and the average particle size thereof ispreferably about 5 nm to about 500 nm. The average particle size isusually measured with a transmission electron microscope.

The backcoat layers may contain non-magnetic inorganic powders such asthe various abrasives described above with respect to the magneticlayers, in addition to the above-mentioned carbon black, in order toenhance the mechanical strength. The content of the non-magneticinorganic powder is preferably 0.1 to 5 parts by weight per 100 parts byweight of carbon black, and more preferably 0.5 to 2 parts by weight.The average particle size of the non-magnetic inorganic powder ispreferably 0.1 μm to 0.5 μm. Too low a content of such a non-magneticinorganic powder is liable to make the mechanical strength of thebackcoat layer insufficient, whereas too high a content tends toincrease the abrasion of a guide in a tape sliding path.

Besides, dispersing agents such as surfactants, lubricants such ashigher fatty acids, fatty acid esters and silicone oil, and othervarious additives may be added if necessary.

Binders, crosslinking agents and solvents used in the backcoat layersmay be the same as those used in the above-mentioned paints for magneticlayers. The content of the binder is preferably 15 parts to 200 parts byweight per 100 parts by weight of the total solid content, and morepreferably 50 parts to 180 parts by weight. Too high a content of thebinder increases friction between the magnetic recording medium and asliding path thereof in excess to deteriorate the running stability,resulting in a tendency to bring about running trouble. The problem ofblocking with the magnetic layer is further encountered. Too low acontent of the binder lowers the strength of the backcoat layer to beliable to deteriorate the running durability.

The thickness of the backcoat layer (after calendaring treatment) is 1.0μm or less, preferably 0.1 μm to 1.0 μm, and more preferably 0.2 μm to0.8 μm. Too thick a thickness of the backcoat layer increases frictionbetween the magnetic recording medium and a sliding path thereof inexcess to tend to deteriorate the running stability, whereas too thin athickness deteriorates the surface properties of the backcoat layer,affected by the surface properties of the magnetic recording medium.Accordingly, when the backcoat layer is thermoset, the roughness on thesurface of the backcoat layer is transferred to the surface of themagnetic recording medium to cause decreases in high frequency output,S/N and C/N. On the other hand, if the backcoat layer is too thin,scratches are produced on the backcoat layer in running of the magneticrecording medium.

Supports used in the present invention include polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), polyamide,polyimide and polyamideimide films containing inorganic compounds suchas oxides and carbonates of Al, Ca, Si and Ti, and organic compoundssuch as fine acrylic resin powders as fillers. Of these, the PET, PENand aromatic polyamide films are preferred, and composite films producedby multilayer coextrusion of two or three kinds of PET and PEN are morepreferred. Further, the supports may previously be subjected to coronadischarge treatment, plasma discharge treatment and/or polymerizationtreatment, coating treatment of adhesion improving agents, dustprooftreatment, and relaxation treatment by heat and/or humidification.

In the present invention, the support may be provided with the magneticlayer on one surface, or with magnetic layers on both surfaces, or withmultiple magnetic layers. In particular, when provided with the magneticlayer only on one surface, the support is preferably provided with thebackcoat layer on the surface opposite to the magnetic layer. Theundercoat layer composed of the non-ferromagnetic material may be formedunder the magnetic layer for improving the surface properties, theadhesive properties and the durability. Further, a protectivelubricating layer such as a lubricant, a plasma-polymerized film or adiamond-like film may be formed on the magnetic layer for lubricatingand protecting the magnetic layer.

Manufacturing Processes of Paints

Processes for manufacturing the magnetic paints of the magneticrecording media of the present invention and the paints for forming themagnetic recording media (backcoat paints and undercoat paints) eachcomprise at least kneading steps, dispersing steps, filtering steps,optional mixing steps given before or after these steps, and storingsteps. In the kneading steps, devices having strong kneading abilitysuch as continuous kneaders, pressure kneaders, high-speed mixers andtwo-roll mills are used, and all or a part of the pigment powders andthe binders are kneaded. Further, in dispersion of the paints, zirconiaor glass beads can be used. In the dispersing steps, dilution dispersionis used in combination so that the solid concentration is graduallydecreased. The individual steps may be each divided into two or morestages, and raw materials may be added in lots at two or more stages.

The filtering steps of the paints are preferably provided after eachproduction of the respective paints. If magnetic powders not dispersedor coagulated, or resins not dissolved exist in the magnetic paints,they become defects in the magnetic paints, which causes increases indropout and in error rate. A main object of the filtering stages is toremove these contaminants in the magnetic paints. Details thereof aredescribed in Japanese Patent Application No. 6-321357 previouslyproposed by the present inventors.

Coating Processes

Elongated film-like non-magnetic supports drawn out of unwind rolls arecoated with the paints while incorporating hardeners therein (or inwhich hardeners have been incorporated) as required, said paints havingbeen filtered through filters having specified filtering precision, byvarious known coating means such as gravure coating, reverse rollcoating and extrusion nozzle coating.

In general, before coating of these coatings, the non-magnetic supportsare treated by various known means such as wet cleaning using water orsolvents, dry cleaning using nonwoven fabrics or ultrafine fiber wovenfabrics as wipers, and non-contact cleaning using compressed air, vacuumor ionized air, for cleaning and surface adjustment. Further, they aresubjected to various known non-contact surface treatments such as coronadischarge, ultraviolet light irradiation and electron beam irradiation,for improving adhesion of the paints to the non-magnetic supports andimproving coating surfaces.

Further, the non-magnetic supports may be coated with undercoatingagents such as aqueous undercoating agents, emulsion undercoating agentsand solvent undercoating agents independently or together with theabove-mentioned surface treatments for improving adhesion in some cases.Technically, in place of the undercoat layers consisting of resinsalone, paints in which non-ferromagnetic inorganic or organic pigmentsare dispersed in binders may be applied as the undercoat layers, and maybe used in combination with the above-mentioned surface treatments. Theplural undercoat layers may be preferably formed at the same time by thewet-on-wet coating process according to extrusion nozzle coating as longas they are not abnormally separated or not repelled from the magneticlayers or the backcoat layers.

Although the magnetic layer is generally formed independently, it isalso possible to provide two or more magnetic layers for giving higherfunctions. In that case, the magnetic layers and the non-ferromagneticlayers may be formed by known methods such as the wet-on-dry coatingprocess and the wet-on-wet coating process.

After such a coating process, various treatments such as smoothing ofwet film surfaces of the magnetic paints formed on the non-magneticsupports and coated film regulation may be usually conducted. Assmoothing means, known processes can be employed such as contactprocesses using films or bars of resins, metals or ceramics andnon-contact processes using vibration caused by magnetic fields orultrasonic waves generated with permanent magnets or electromagnets.They may be used alone or in combination depending on desiredcharacteristics.

Orientation Processes

After formation of the magnetic layers, it is necessary to orient themagnetic particles in the layers by application of magnetic fields. Theorientation direction thereof may be longitudinal, vertical or obliqueto the running direction of the magnetic recording media. In order toorient the magnetic particles in the specified direction, it ispreferred to apply a magnetic field of 1000 G or more in the specifieddirection with a permanent magnet such as a ferrite magnet or a rareearth magnet, an electromagnet or a solenoid, or to use these pluralmagnetic field generating means in combination. Further, proper dryingmay be conducted before the orientation or simultaneously therewith sothat the highest orientation is obtained after the orientation. In thecase of floppy disks, the orientation of the magnetic powders naturallyorientated by coating may be reduced as low as possible with a permanentmagnet, an electromagnet or a solenoid.

Drying Processes

The magnetic paints thus treated after coating are dried and fixed byknown drying and evaporating means such as hot air, far infrared rays,electric heaters and vacuum evaporators usually mounted in dryingfurnaces, or known curing devices such as ultraviolet lamps andradiation irradiation devices.

The drying temperature ranges from room temperature to 300° C., and maybe suitably selected depending on the heat resistance of thenon-magnetic supports, and the kind and the concentration of solvents.Further, the temperature gradient may be given into the drying furnaces,and the gas atmospheres in the drying furnaces may be general air orinert gases.

Drying with ultraviolet lamps or radiation irradiation devices causesthe curing reaction. Considering the after treatment, therefore, otherdrying means are preferably used as much as possible.

Irradiation of ultraviolet rays or radiation with solvents contained issometimes accompanied by ignition or smoking. Accordingly, also in thiscase, other drying means may be used in combination as much as possible.

Calendaring Processes

After drying of the magnetic layers like this, calendaring treatment iscarried out as surface smoothing treatment if necessary. Calendaringrolls include combinations (3 to 7 stages) of metal rolls andheat-resistant plastic rolls made of epoxy resins, polyesters, nylon,polyimides, polyamides and polyimideamides (in which inorganic compoundssuch as carbon and metals may be kneaded), and combinations of metalrolls. The treating temperature is preferably 70° C. or more, and morepreferably 80° C. or more. The linear pressure is preferably 200 kg/cmor more, and more preferably 300 kg/cm or more. The speed ranges from 20m/minute to 700 m/minute.

After Treatment

After the calendaring treatment, thermosetting treatment at 40° C. to80° C. and/or electron beam irradiation treatment may be applied inorder to enhance curing of the magnetic layers, the backcoat layers andthe non-ferromagnetic layers.

Then, formation of tapes or disks of specified shape with slitters orpressing machines is followed by secondary processing such as grindingor cleaning on magnetic surfaces and/or backcoat surfaces to preparemagnetic recording media.

The present invention will be further illustrated with reference to thefollowing examples, which are, however, not to be construed as limitingthe invention.

The characteristics of the magnetic tapes were measured by the followingmethods.

Gloss

The gloss (%) at a measurement angle of 60° was measured with a GM-3Ddigital gloss meter manufactured by Murakami Shikisai Gijutsu Kenkyusho.The optical conditions described in JIS-Z-8741 were employed.

Electromagnetic Characteristics

7 MHz Output

A 7-MHz signal was recorded on a tape sample, and when this signal wasreproduced, a 7-MHz reproduced output was measured with an oscilloscopeas a relative value taking the 7-MHz output of Comparative Example 7 as0 dB.

Y-C/N

A 7-MHz signal was recorded on a tape sample, and when this signal wasreproduced, a noise generated at 6 MHz was measured with a spectrumanalyzer to determine the ratio of the reproduced signal to this noiseas a relative value taking the Y-C/N of Comparative Example 8 as 0 dB.

Deck Used: BR-S711 (S-VHS format VTR) manufactured by JVC Co.

Running Durability

The shuttle running of a tape sample was repeated 100 times for 50 reelson a VTR under the circumstances of 20° C. and 60% RH, and theoccurrence of running troubles such as running stoppage and headclogging were observed to evaluate the running durability based on thefollowing criterion.

Deck Used: BR-S711 (S-VHS format VTR) manufactured by JVC Co.

O: No troubles occurred.

Δ: Troubles occurred for one reel.

x: Troubles occurred for two or more reels.

Still Durability

Each tape sample was reproduced in the still mode, and the time taken toreduce a 7-MHz reproduced output by 6 dB was measured.

Deck Used: BR-S711 (S-VHS format VTR) manufactured by JVC Co.

EXAMPLES 1 TO 17 AND COMPARATIVE EXAMPLES 1 TO 7

Production of Vinyl Chloride Copolymers

(1) Production of Vinyl Chloride-Vinyl Acetate-VinylAlcohol-N,N-Dimethylethanolamine-Modified Vinyl Unit-Maleic AcidCopolymer

An autoclave equipped with a stirrer was charged with methanol, vinylchloride, vinyl acetate, maleic acid, di(2-ethylhexyl)peroxydicarbonateand partially saponified polyvinyl alcohol, and the temperature waselevated to 60° C. with stirring under an atmosphere of nitrogen gas toinitiate the reaction, followed by further continuous injection of vinylchloride to conduct the copolymerization reaction. After release of theresidual pressure of the autoclave and cooling, a copolymer slurry wastaken out and filtered, followed by washing with methanol three timesand further with deionized water twice, thus obtaining a vinylchloride-vinyl acetate-maleic acid copolymer powder. This copolymer,methanol, N,N-dimethylethanolamine and sodium hydroxide were added to areactor equipped with a cooling pipe, and allowed to react at 40° C.Then, the reaction product was cooled, and acetic acid was added theretoto neutralize unreacted sodium hydroxide. The resulting product waswashed with methanol three times and further with deionized water twice,followed by filtering and drying to obtain a vinyl chloride-vinylacetate-vinyl alcohol-maleic acid-N,N-dimethylethanolamine-modifiedvinyl unit copolymer powder. Copolymers different in weight ratio ofmonomers shown in Table 1 were produced by changing the amounts of therespective monomer units or amine compounds.

(2) Production of Vinyl Chloride-Vinyl Acetate-VinylAlcohol-N,N-Dimethylethanolamine-Modified Vinyl Unit-SodiumMethacrylsulfonate Copolymer

In the above-mentioned process (1), sodium methacrylsulfonate was usedinstead of maleic acid to produce a copolymer.

(3) Production of Vinyl Chloride-Vinyl Acetate-VinylAlcohol-N,N-Dimethylethanolamine-Modified Vinyl Unit-2-AcidPhosphooxyethyl Methacrylate Copolymer

In the above-mentioned process (1), 2-acid phosphooxyethyl methacrylatewas used instead of maleic acid to produce a copolymer.

(4) Production of Vinyl Chloride-Vinyl Acetate-Vinyl Alcohol Copolymer

An autoclave equipped with a stirrer was charged with methanol, vinylchloride, vinyl acetate, di(2-ethylhexyl) peroxydicarbonate andpartially saponified polyvinyl alcohol, and the temperature was elevatedto 60° C. with stirring under an atmosphere of nitrogen gas to initiatethe reaction, followed by further continuous injection of vinyl chlorideto conduct the copolymerization reaction. After release of the residualpressure of the autoclave and cooling, a copolymer slurry was taken outand filtered, followed by washing with methanol three times and furtherwith deionized water twice. Then, the resulting product was filtered anddried to obtain a vinyl chloride-vinyl acetate copolymer powder. Thiscopolymer, methanol and sodium hydroxide were added to a reactorequipped with a cooling pipe, and allowed to react at 40° C. Then, thereaction product was cooled, and acetic acid was added thereto toneutralize unreacted sodium hydroxide. The resulting product was washedwith methanol three times and further with deionized water twice,followed by filtering and drying to obtain a vinyl chloride-vinylacetate-vinyl alcohol copolymer powder.

(5) Production of Vinyl Chloride-Vinyl Acetate-Vinyl Alcohol-Maleic AcidCopolymer

An autoclave equipped with a stirrer was charged with methanol, vinylchloride, vinyl acetate, maleic acid, di(2-ethylhexyl)peroxydicarbonateand partially saponified polyvinyl alcohol, and the temperature waselevated to 60° C. with stirring under an atmosphere of nitrogen gas toinitiate the reaction, followed by further continuous injection of vinylchloride to conduct the copolymerization reaction. After release of theresidual pressure of the autoclave and cooling, a copolymer slurry wastaken out and filtered, followed by washing with methanol three timesand further with deionized water twice. Then, the resulting product wasfiltered and dried to obtain a vinyl chloride-vinyl acetate-maleic acidcopolymer powder. This copolymer, methanol and sodium hydroxide wereadded to a reactor equipped with a cooling pipe, and allowed to react at40° C. Then, the reaction product was cooled, and acetic acid was addedthereto to neutralize unreacted sodium hydroxide. The resulting productwas washed with methanol three times and further with deionized watertwice, followed by filtering and drying to obtain a vinyl chloride-vinylacetate-vinyl alcohol-maleic acid copolymer powder.

(6) Production of Vinyl Chloride-Vinyl Acetate-VinylAlcohol-N,N-Dimethylethanolamine-Modified Vinyl Unit Copolymer

An autoclave equipped with a stirrer was charged with methanol, vinylchloride, vinyl acetate, di(2-ethylhexyl)peroxydicarbonate and partiallysaponified polyvinyl alcohol, and the temperature was elevated to 60° C.with stirring under an atmosphere of nitrogen gas to initiate thereaction, followed by further continuous injection of vinyl chloride toconduct the copolymerization reaction. After release of the residualpressure of the autoclave and cooling, a copolymer slurry was taken outand filtered, followed by washing with methanol three times and furtherwith deionized water twice. Then, the resulting product was filtered anddried to obtain a vinyl chloride-vinyl acetate copolymer powder. Thiscopolymer, methanol, N,N-dimethylethanolamine and sodium hydroxide wereadded to a reactor equipped with a cooling pipe, and allowed to react at40° C. Then, the reaction product was cooled, and acetic acid was addedthereto to neutralize unreacted sodium hydroxide. The resulting productwas washed with methanol three times and further with deionized watertwice, followed by filtering and drying to obtain a vinyl chloride-vinylacetate-vinyl alcohol-N,N-dimethylethanolamine-modified vinyl unitcopolymer powder.

Production of Polyesterpolyurethane Resins

Dimethyl terephthalate, dimethyl isophthalate, dimethyl5-sodiumsulfoisophthalate, ethylene glycol, neopentyl glycol, zincacetate and sodium acetate were placed in a reactor equipped with athermometer, a stirrer and a partial reflux condenser, andtransesterification was conducted at 140° C. to 220° C. for 3 hours.Then, the reaction system was evacuated to 5 mm Hg for 20 minutes, andthe temperature thereof was elevated to 250° C. in the meantime.Further, the polycondensation reaction was conducted at 250° C. at 0.1mm Hg for 60 minutes to obtain polyesterdiol (1), which had thecomposition of terephthalic acid/isophthalicacid/5-sodiumsulfoisophthalic acid/ethylene glycol/neopentyl glycol in amolar ratio of 49/49/2/50/50. In a similar manner, polyesterdiol (2) wasobtained. The composition thereof was adipicacid/1,4-butanediol/neopentyl glycol in a molar ratio of 100/75/25.

Toluene, methyl isobutyl ketone, polyesterdiol (1), polyesterdiol (2),4,4'-diphenylmethane diisocyanate, neopentyl glycol and dibutyltindilaurylate were placed in a reactor equipped with a thermometer, astirrer and a partial reflux condenser, and allowed to react at 70° C.to 90° C. for 8 hours to obtain polyesterpolyurethane resin 1 andpolyesterpolyurethane resin 2 having molecular weights shown in Table 1.

Preparation of Magnetic Paints

    ______________________________________    Preparation of Binder Solutions    ______________________________________    Vinyl chloride copolymer (vinyl chloride-vinyl                               50     parts    acetate-vinyl alcohol-N,N-dimethylethanolamine-modified    vinyl unit-maleic acid copolymer, monomer weight ratio:    shown in Table 1, average degree of polymerization:    shown in Table 1    Polyesterpolyurethane resin 1 (containing --SO.sub.3 Na                               20     parts    groups, Mn: shown in Table, Tg: 20° C.)    Polyesterpolyurethane resin 2 (containing --SO.sub.3 Na                               30     parts    groups, Mn: shown in Table, Tg: 20° C.)    Methyl ethyl ketone (MEK)  200    parts    Toluene                    100    parts    Cyclohexane                100    parts    ______________________________________

The above-mentioned compositions were placed in an agitator, and mixedby stirring for 6 hours to form a binder solution. The above-mentionedbinder solution was filtered by circulation through a Depth type filterhaving a 95% cut filtration accuracy of 5.0 μm for 8 hours.

    ______________________________________    Kneading-Dispersing Treatment    ______________________________________    Co-coated iron oxide magnetic powder (Hc: 8000e, δs:                               700    parts    70 emu/g, specific surface area: 45 m.sup.2 /g)    α-Al.sub.2 O.sub.3 (HIT-50 manufactured by Sumitomo Chemical                               35,    parts    Ltd.)    Cr.sub.2 O.sub.3 (U-1 manufactured by Nippon Chemical Industrial                               35     parts    Co., Ltd.)    Binder solution            250    parts    ______________________________________

The above-mentioned compositions were placed in a pressure kneader, andkneaded for 2 hours. After kneading, the following compositions wereadded thereto to adjust the viscosity to the optimum for dispersingtreatment.

    ______________________________________    Binder solution      250    parts    MEK                  200    parts    Toluene              200    parts    Cyclohexane          200    parts    ______________________________________

After mixing treatment, dispersing treatment was performed in a sandmill.

    ______________________________________    Adjustment of Viscosity    ______________________________________    Stearic acid         3      parts    Myristic acid        3      parts    Butyl stearate       3      parts    MEK                  200    parts    Toluene              200    parts    Cyclohexane          200    parts    ______________________________________

The above-mentioned compositions were placed in an agitator, and mixedby stirring for 1 hour to prepare a viscosity adjusting solution. Theabove-mentioned viscosity adjusting solution was filtered by circulationthrough a Depth type filter having a 95% cut filtration accuracy of 1.2μm for 8 hours.

After mixing of the viscosity adjusting solution filtered by circulationwith the slurry dispersed, dispersing treatment was carried out in asand mill to adjust the viscosity* to 40 cp, thereby forming a magneticpaint. The above-mentioned paint was filtered by circulation through aDepth type filter having a 95% cut filtration accuracy of 1.2 μm for 8hours.

*) Method for measuring the viscosity: the viscosity was determined at aliquid temperature of 20° C. at a shear rate of 3000 sec⁻¹ with anMR-300 soliquid meter manufactured by Rheology Co.

Final Paints

To 100 parts by weight of each paint after filtration, 0.8 part byweight of an isocyanate compound (Coronate L manufactured by NipponPolyurethane Co., Ltd.) was added, and mixed by stirring to prepare eachmagnetic layer paint.

Preparation of Backcoat Paint

    ______________________________________    Preparation of Binder Solution    ______________________________________    Vinyl chloride copolymer (vinyl chloride/vinyl acetate/                               35     parts    vinyl alcohol/N,N-dimethylethanolamine-modified    vinyl unit/maleic acid copolymer (91/3.0/5.0/0.5/0.5/0.5%    by weight, average degree of polymerization: 400)    Polyesterpolyurethane resin (containing --SO.sub.3 Na                                35    parts    groups, Mn: 40000, Tg: 20° C.)    MEK                        100    parts    Toluene                    80     parts    Cyclohexane                100    parts    ______________________________________

The above-mentioned compositions were placed in an agitator, and mixedby stirring for 6 hours to form a binder solution. The above-mentionedbinder solution was filtered by circulation through a Depth type filterhaving a 95% cut filtration accuracy of 5.0 μm for 8 hours.

    ______________________________________    Kneading-Dispersing Treatment    ______________________________________    Carbon black (#47B manufactured by Mitsubishi Chemical                               100    parts    Corporation)    Carbon black (MT-CI manufactured by Colombian Carbon                               1      part    Co.)    α-Fe.sub.2 O.sub.3   1      part    Binder solution            130    parts    ______________________________________

The above-mentioned compositions were placed in a pressure kneader, andkneaded for 2 hours. After kneading, the following compositions wereadded thereto to adjust the viscosity to the optimum for dispersingtreatment.

    ______________________________________     Binder solution     45     parts    MEK                  70     parts    Toluene              55     parts    Cyclohexane          70     parts    ______________________________________

After mixing treatment, dispersing treatment was performed in a sandmill.

    ______________________________________    Adjustment of Viscosity    ______________________________________    Binder solution      175    parts    Stearic acid         1      parts    Myristic acid        1      parts    Butyl stearate       1      parts    MEK                  200    parts    Toluene              250    parts    Cyclohexane          200    parts    ______________________________________

The above-mentioned compositions were placed in an agitator, and mixedby stirring for 1 hour to prepare a viscosity adjusting solution. Theabove-mentioned viscosity adjusting solution was filtered by circulationthrough a Depth type filter having a 95% cut filtration accuracy of 1.2μm for 8 hours.

After mixing of the viscosity adjusting solution filtered by circulationwith the slurry dispersed, dispersing treatment was carried out in asand mill to adjust the viscosity* to 10 cp, thereby forming a backcoatpaint. The above-mentioned paint was filtered by circulation through aDepth type filter having a 95% cut filtration accuracy of 1.2 μm for 8hours.

*) Method for measuring the viscosity: the viscosity was determined at aliquid temperature of 20° C. at a shear rate of 3000 sec⁻¹ with anMR-300 soliquid meter manufactured by Rheology Co.

Final Paint

One part by weight of an isocyanate compound ((Coronate L manufacturedby Nippon Polyurethane Co., Ltd.) was added to 100 parts by weight ofthe paint after filtration, and mixed by stirring to prepare a backcoatlayer paint.

Preparation of Magnetic Tapes

A surface of each non-magnetic support (polyethylene terephthalate filmhaving a thickness of 14.8 μm) was coated with each magnetic layerpaint, followed by orientation treatment. The film coated was dried, andthen treated with a 7-stage calender at a temperature of 110° C. at alinear pressure of 280 kg/cm. The film thickness of the magnetic layersof all samples after calendaring processing was 2.0 μm. Further, theback of the non-magnetic support was coated with the backcoat layerpaint. After drying, the film coated was treated with a 7-stage calenderat a temperature of 110° C. at a linear pressure of 280 kg/cm. The filmthickness of the backcoat layers of all samples after calendaringprocessing was 0.5 μm.

Each film coated was cured for 24 hours in an oven heated at 60° C.,then slit to a width of 1/2 inch, and incorporated in a cassette toprepare a magnetic tape sample. The characteristics of the resultingmagnetic tape samples are as shown in Table 1.

EXAMPLES 18 AND 19

Tape samples were prepared in the same manner as with Example 2 with theexception that the vinyl chloride-vinyl acetate-vinylalcohol-N,N-dimethylethanolamine-modified vinyl unit-sodiummethacrylsulfonate copolymer obtained in vinyl chloride copolymerproduction (2) described above and the vinyl chloride-vinylacetate-vinyl alcohol-N,N-dimethylethanol-amine-modified vinylunit-2-acid phosphooxyethyl methacrylate copolymer obtained in vinylchloride copolymer production (3) described above were each used as thevinyl chloride resins contained in the magnetic paints. The resultingtape samples had effects similar to those of Example 2.

The magnetic recording medium of the present invention comprises thenon-magnetic support and the magnetic layer formed thereon which ismainly composed of the magnetic powder and the binder, wherein saidbinder comprises as essential ingredients three of

i) the vinyl chloride copolymer having amine-modified vinyl units andacidic functional group-containing vinyl units in a molecule,

ii) the polyurethane resin having a number average molecular weight of5000 to 25000 (Mn 1), and

iii) the polyurethane resin having a number average molecular weight ofhigher than 25000 to 100000 (Mn 2), thereby improving the dispersibilityand the dispersion stability of the magnetic paint. As a result, themagnetic recording medium having the good electromagneticcharacteristics and excellent in the running durability is obtained.

That is, the magnetic recording medium containing no polyurethane resinof Mn 1 is poor in electromagnetic characteristics, and the magneticrecording medium containing no polyurethane resin of Mn 2 is poor indurability and still characteristics. Further, no inclusion ofamine-modified vinyl units in the vinyl chloride copolymer results indeteriorated dispersibility and electromagnetic characteristics, and noinclusion of acidic functional group-containing vinyl units alsosimilarly results in deteriorated dispersibility and electromagneticcharacteristics.

    TABLE 1      - Vinyl Chloride Copolymer  Gloss (%) Electromagnetic      Example and  Average  Polyurethane 1 Polyurethane 2  Viscosity     Characteristics  Still      Comparative  Monomer Weight Degree Compounding  Compounding  Compounding      Dispersing Adjusting  7M out Y-C/N Running Durability      Example Ratio* (WT %) of Polym. Ratio (WT %) Mn 1 Ratio (WT %) Mn 2     Ratio (WT %) Step (A) Step (B) (B) - (A) put (dB) (dB) Durability     (min)      Example 1 91/3.0/5.0/0.5/0.5 400 50 23000 20 30000 30 120 121 +1 +2.0     +2.1 ◯ 110      Example 2 91/3.0/5.0/0.5/0.5 400 50 23000 20 50000 30 118 122 +4 +1.9     +1.8 ⊚ >120      Example 3 91/3.0/5.0/0.5/0.5 400 50 23000 20 80000 30 117 120 +3 +1.7     +1.9 ⊚ >120      Example 4 91/3.0/5.0/0.5/0.5 400 50 12000 20 50000 30 122 125 +3 +2.1     +2.1 ⊚ >120      Example 5 91/3.0/5.0/0.5/0.5 400 50 8000 20 50000 30 124 124 0 +2.2     +2.0 ◯ 110      Example 6 91/3.0/5.0/0.5/0.5 400 50 23000 30 50000 20 120 120 0 +2.0     +2.1 ⊚ 110      Example 7 91/3.0/5.0/0.5/0.5 400 30 23000 30 50000 40 119 119 0 +1.9     +1.9 ⊚ >120      Example 8 91/3.0/5.0/0.5/0.5 400 70 23000 10 50000 20 125 127 +2 +2.3     +2.4 ⊚ >120      Example 9 91/3.0/5.0/0.5/0.5 300 50 23000 20 50000 30 127 129 +2 +2.5     +2.4 ◯ >120      Example 10 91/3.0/5.0/0.5/0.5 500 50 23000 20 50000 30 116 119 +3 +1.7     +1.8 ⊚ >120      Example 11 91/3.0/4.5/1.0/0.5 400 50 23000 20 50000 30 119 123 +4 +2.2     +2.1 ⊚ >120      Example 12 91/3.0/4.0/1.5/0.5 400 50 23000 20 50000 30 121 124 +3 +2.2     +2.3 ⊚ >120      Example 13 91/3.0/4.5/0.5/1.0 400 50 23000 20 50000 30 123 125 +2 +2.4     +2.3 ⊚ >120      Example 14 91/3.0/4.0/0.5/1.5 400 50 23000 20 50000 30 124 125 +1 +2.4     +2.4 ⊚ >120      Example 15 91/3.0/5.0/0.5/0.5 400 50 5000 20 50000 30 124 124 0 +2.3     +2.2 ◯ 110      Example 16 91/3.0/5.0/0.5/0.5 400 50 25000 20 50000 30 118 121 +3 +1.8     +1.8 ⊚ >120      Example 17 91/3.0/5.0/0.5/0.5400 50 23000 20 100000 30 116 120 +4 +1.7     +1.8 ⊚ >120      C. Ex. 1 91/3.0/5.0/0.5/0.5 400 50 23000 20 120000 30 110 111 +1 +0.1     +0.2 ⊚ >120      C. Ex. 2 91/3.0/5.0/0.5/0.5 400 50 23000 50 -- -- 125 127 +2 +2.1 +2.0     x 50      C. Ex. 3 91/3.0/5.0/0.5/0.5 400 50 -- -- 50000 50 109 111 +2 0.0 0.0     ⊚ >120      C. Ex. 4 91/3.0/5.0/0.5/0.5 400 50 3000 20 50000 50 125 120 -5 +1.8     +1.7 x 40      C. Ex. 5 91/3.0/6.0/0/0 400 50 23000 20 50000 30 95 95 0 -0.7 -1.0     ⊚ >120      C. Ex. 6 91/3.0/5.5/0/0.5 400 50 23000 20 50000 30 102 104 +2 -0.3 -0.4     ⊚ >120      C. Ex. 7 91/3.0/5.5/0.5/0 400 50 23000 20 50000 30 100 90 -10 -0.8 -0.9     ⊚ >120     *)Monomer weight ratio: vinyl chloride/vinyl acetate/vinyl     alcohol/dimethylethanolamine modified vinyl unit/maleic acid     C. Ex: Comparative Example     Average Degree of Polym.: Average Degree of Polymerization

What is claimed is:
 1. A magnetic recording medium comprising anon-magnetic support and a magnetic layer formed thereon which is mainlycomposed of a magnetic powder and a binder, wherein said binder is amixture ofi) a vinyl chloride copolymer having amine-modified vinylunits and acidic functional group-containing vinyl units in a molecule,ii) a polyurethane resin having a number average molecular weight of5000 to 25000 (Mn 1), and iii) a polyurethane resin having a numberaverage molecular weight of higher than 25000 to 100000 (Mn 2).
 2. Themagnetic recording medium according to claim 1, wherein said vinylchloride copolymer has an average degree of polymerization of 200 to800.
 3. The magnetic recording medium according to claim 1, wherein thecontent of the amine-modified vinyl units contained in the vinylchloride copolymer is 0.03% to 2.3% by weight, and the content of theacidic functional group-containing vinyl units is 0.05% to 1.8% byweight.
 4. The magnetic recording medium according to claim 1, wherein aprimary amine, a secondary amine or a tertiary amine is contained as theamine-modified vinyl unit, and a carboxyl group, a sulfonic acid group,a sulfuric acid group, a phosphoric acid group or a phosphonic acidgroup is contained as the acidic functional group-containing vinyl unit.5. The magnetic recording medium according to claim 1, wherein saidpolyurethane resin having a number average molecular weight of 5000 to25000 (Mn 1) is a polyester-polyurethane resin, a polyetherpolyurethaneresin or a polycarbonatepolyurethane resin.
 6. The magnetic recordingmedium according to claim 1, wherein said polyurethane resin having anumber average molecular weight of higher than 25000 to 100000 (Mn 2) isa polyesterpolyurethane resin, a polyetherpolyurethane resin or apolycarbonatepolyurethane resin.
 7. The magnetic recording mediumaccording to claim 1, wherein said vinyl chloride copolymer is containedin an amount of 20% to 80% by weight based on the total amount of thebinder, said polyurethane resin of Mn 1 is contained in an amount of 10%to 50% by weight, said polyurethane resin of Mn 2 is contained in anamount of 10% to 50% by weight, and the total amount of saidpolyurethane resin of Mn 1 and said polyurethane resin of Mn 2 is 20% to80% by weight.